1. Field of the Invention
The present invention relates to an electrostatic image forming apparatus such as a printer, a copying machine, a facsimile system, and the like. More particularly, the invention relates to a structure for transporting a toner supplied from a toner supply cartridge into a developing chamber.
2. Description of the Related Art
A conventional image forming apparatus employs a dry developing method, in which development is achieved by supplying toner to a developing region on the surface of a photosensitive drum. In such an apparatus, the surface of the photosensitive drum is uniformly charged by a charger, then light beams corresponding to image data are projected onto the surface of the photosensitive drum. The light beams form an electrostatic latent image on the surface of the photosensitive drum. Next, a toner, in the form of fine particles magnetized by a developing device, are developed onto the electrostatic latent image to form a toner image which is transferred onto a recording sheet.
For example, in an image forming apparatus disclosed in Japanese Patent Laid-open No. 64-52182, a developing device (developing tank) adjacent a photosensitive drum includes a developing sleeve incorporating a magnetic roller in its inner portion. The developing sleeve carries toner and a carrier on its circumferential surface. A toner supply roller supplies toner to the developing sleeve. In a toner supply chamber (make-up chamber), which is separated from the developing chamber by a partition wall, a rotating agitating member supplies the toner into the developing chamber. The toner supply chamber stores new, unused toner, which is supplied from a toner source. The toner in the toner supply chamber is sent over the partition wall and into the developing chamber by rotation of the agitating member.
As the recording operation is repeatedly performed, as described above, toner is consumed and it becomes necessary to supply additional toner to the toner supply chamber. To add additional toner, the entire process unit or the whole casing of the developing device may be replaced, or a lid on the toner supply chamber may be opened and toner from a toner make-up box may be emptied into the toner supply chamber to replenish the toner supply.
Replacing the entire process unit or the entire developing device, however, can be very expensive. In order to prolong the interval between replacement of the developing device to reduce the need for replacement, it is necessary to enlarge the toner supply chamber so that a larger amount of toner can be stored. The larger toner supply chamber, however, causes the apparatus as a whole to become larger, which can be undesirable.
On the other hand, when toner from a toner make-up box is added to the toner supply chamber, toner can be easily scattered around and may frequently soil hands and clothes.
In order to solve such problems it is known to provide a member at the side of a toner supply chamber, which allows a toner cartridge containing toner to be detachably attached to the toner supply chamber. Using this system, a replacement operation may be performed wherein a new toner cartridge is attached to the developing device, and an aperture between the cartridge and the toner supply chamber is opened so that the cartridge communicates with the toner supply chamber. Toner from the toner cartridge may thereby be supplied to the toner supply chamber as the recording operation is carried on, and the chance of scattering toner is greatly reduced.
In the above-described toner cartridge system, in order to prevent toner from spilling out of the cartridge while the cartridge is handled, it is necessary to seal the aperture on the cartridge with a shutter. Since it is very difficult to ensure a good seal of the shutter when the aperture is large, the aperture is formed to have the minimum required size in a central portion of the cartridge in its longitudinal direction.
In the above toner cartridge system, toner is discharged into the toner supply chamber from the aperture in the central portion of the toner cartridge. The toner in the toner supply chamber is sent over a partition wall and into the developing chamber by an agitating member. Under such conditions, the pushing force on the toner from the toner cartridge is easily transmitted to the central region of the toner in the toner supply chamber, and the toner in the central region of the toner supply chamber advances straight from the aperture on the toner cartridge into the developing chamber. However, the pushing force on the toner from the toner cartridge is not easily transmitted to the toner present at side portions of the toner supply chamber, and therefore the toner at the side portions cannot easily go over the partition wall. Accordingly, toner is not well supplied to the end portions of the developing sleeve, and hence to the photosensitive drum. As a result, the recording density on a recording sheet may be too light at the side portions. If this condition persists, clogging of the toner may begin at the side portions, and the flow of the toner may gradually become worse, even in the central portion. As a result, the recording density may become insufficient across all of the recording sheet.
If the height of the partition wall is decreased, toner at the side portions is able to easily go over the partition wall, and stagnation of the toner can be prevented. However, toner in the central portion receives less resistance, and too much toner is sent from the central portion of the toner supply chamber into the developing chamber. As a result, an imbalance of the density of the resulting image may occur.